In a traditional ISDN basic access, a two-wire data link is exclusively used for the ISDN service (see FIG. 1). To be able to make better use of existing two-wire data links between a network node and a network termination at the subscriber, a number of services are transmitted simultaneously on one link, for example interleaved in a frame structure, in DSL (Digital Subscriber Line) links having a higher transmission rate. For the discussions following, data transport by means of an SDSL (Symmetric Digital Subscriber Line) frame is considered. It is known that within this frame, for example, the data of one or more broadband links and of one or more ISDN or one or more conventional telephony links can be transmitted. It is also known that, for these links, the information content of the operational bits can be transmitted in the eoc channel present in the SDSL frame.